Wind turbine acoustics - Wind Power Generation and Wind Turbine Design - page 155

⎛

2

⎞

p '

L =× ⎜ 10 log

⎟

( 3)

p

2
ref

⎝ p ⎠

10 5 Pa. The sound pressure level can be
expressed also in terms of the sound intensity, which is defi ned as the sound power
transmitted through unit area and far from the source can be written as

where the reference pressure is p 5 = 2 ×

−

2

p '

I =

(4)

r c

00

where r 0 and c 0 are the undisturbed air density and sound speed, respectively. Thus
the sound pressure level can be written as

⎛ ⎞

I

L =× ⎜ 10 log

⎟ (5 )

p

⎝ I ⎠

ref

where the reference sound intensity is I ref = 10 − 12 W/m 2 . To characterize the total
power of a sound source, one has to integrate the sound intensity over a closed
surface surrounding the source

∫

PI = d S

( 6 )

S

Similarly to the sound pressure level, one can defi ne the sound power level as

⎛ ⎞

P

L =× ⎜ 10 log

⎟ (7 )

p

⎝ P ⎠

ref

where the reference power level is P ref = 10 − 12 W. Thus for a point source with
spherical wave spreading the sound intensity at a distance kR is I / k 2 , I being the
sound intensity at a distance R from the source (see Fig. 1). It is important to note,
that the sound power level, L w , is a characteristic of the sound source, it refl ects

Figure 1: Illustration of sound intensity.